The invention relates to a method of checking the authenticity of a digital electric circuit arrangement, in particular an integrated circuit arrangement, comprising a data exchange between the electric circuit arrangement and an external device.
For example, U.S. Pat. No. 4,904,851 discloses a system in which a data exchange takes place between an external device, for example a card terminal, and an electric circuit, for example in a card comprising an IC, which data exchange serves to determine whether the user of the electrical card is authorized to perform actions via the terminal. In this known system, a random number is transferred from the terminal to the card, where it is subjected to a predetermined function. Also in the terminal, the same number is subjected to the same function. The results are compared and only if they correspond to each other, the user of the card is given an authorization. Consequently, in such systems the electric circuit arrangement provided in the card is used to perform a predetermined function so as to establish the authenticity of the card.
It is further known that, in the manufacture of electric circuit arrangements comprising a plurality of individual circuits, these circuits, contrary to their normal function, are connected in series. In the manufacture, this series connection is used to check whether the circuits in the electric circuit arrangement function completely and properly.
In systems such as those known from U.S. Pat. No. 4,904,851, there is the problem that copies of the electric circuit arrangement provided on the card exist, which copies perform the same function, so that the external device, in this case the terminal, gives the user authorization although the user is not authorized because the electric circuit arrangement on the card is only a copy. Although this copy fulfills the function, that is it subjects the random number which it has received to the proper function and hence supplies the correct result, it is not authorized to use the terminal because it is only a copy, i.e. a forgery.
Therefore, it is an object of the invention to provide a method by means of which such forged cards, which in their normal function fulfill the same functions as a real card, can be checked for authenticity, i.e. a method which enables forged cards to be identified even when, in their normal function, they perform all the right functions.
In accordance with the invention, this object is achieved in that a concatenation of at least a few circuit-technical elements of the electric circuit arrangement is formed, which deviates from the normal, intended use of the circuit arrangement, in that a digital random value generated by the external device and transferred to the electric circuit arrangement is modified in said circuit arrangement by the concatenated elements and transferred to the external device, in that the external device compares the modified value with a check value assigned to the random value transferred to the electric circuit arrangement, and in that the authenticity of the electric circuit arrangement is recognized only when the modified value and the check value correspond to each other.
Contrary to the normal function of the electric circuit arrangement, in the method in accordance with the invention, at least a few circuit-technical elements are concatenated. Also other elements, for example purely functional elements, may be included in this concatenation operation. It is of essential importance that the concatenation differs from the standard function of the circuit arrangement and that it is used to change a random value supplied by an external source. The structure of the electric circuit arrangement or of the circuit-technical elements influences said change, i.e. the type of change of the random value is governed by the structure of the circuit. Consequently, in addition to the normal function of the circuit, which is capable of subjecting a random value supplied by an external source to a specific function, a further check is carried out to determine whether the structure of the card corresponds to that of a real card or whether it comprises other circuit-technical elements or differently structured circuits, which cause a random value supplied by an external source, which value is changed by concatenating the circuit-technical elements, to be changed in a manner which differs from that of a real, unadulterated card.
The random value modified by the concatenated elements is retransferred to the external device, which compares the value with a check value, which is stored in the external device and assigned to the random value previously transferred to the electric circuit arrangement by the external device. Only when these values correspond to each other, it can be safely assumed that the card is unadulterated and authentic.
Dependent upon the number of circuit-technical elements or other elements involved in the concatenation, the exactness of this authenticity test can be influenced.
Electric circuit arrangements which are copies and which satisfactorily perform their normal functions can still be recognized by the method in accordance with the invention when they demonstrate a different hardware structure and a different functional structure. This enables copies which are not fully identical with the originals, which applies to almost all copies, to be recognized.
The invention can be advantageously used, for example, for security functions for Smart Card ICs, for chip identifications and, in general, for authenticity checks.
In accordance with an embodiment of the invention, the concatenation advantageously not only includes circuit-technical elements but also logic elements of the electric circuit arrangement, because this causes the certainty with which the authenticity of the electric circuit arrangement is recognized to be increased.
The same effect is achieved in that, in accordance with a further embodiment of the invention, a sufficiently large number of circuit-technical elements and logic elements of the electric circuit arrangement is included in the concatenation, so that the modified value constitutes a definite authenticity feature.
Advantageously, as described in claim 6, also all circuit-technical and logic elements which are essential for the functioning should be included in the concatenation.
The circuit-technical elements and, advantageously, also the logic elements can be advantageously concatenated serially, as claimed in claim 4, or, as claimed in claim 5, concatenated in an exclusive-OR-operation.
The measure as claimed in claim 7 increases the certainty of recognition in that the modified value is calculated and checked a number of times.
The same advantage is achieved by the embodiment as claimed in claim 8, in that by using a number of random numbers and comparing the modified values generated by these random numbers, an accidentally correct result can be recognized as being false because the other checks yield false modified values.
A concatenation of elements of the electric circuit arrangement as described, for manufacturing purposes, in the opening paragraph can also advantageously be used for the method in accordance with the invention. In this method, however, said concatenation is not used to test whether the electric circuit arrangement functions perfectly, as in the case of the concatenation for the manufacture, but instead it serves here to check whether the circuit is duly constructed, that is whether the circuit is authentic and not a copy.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.